In a radiotelephone communication system, a communication link via an RF channel is established between a mobile station, or subscriber unit, and a source base station. As a mobile station moves out of range of the source base station, the signal quality will degrade until the communication link would ultimately be broken, or the call "dropped". To avoid loss of the communication link resulting from a dropped call, the communication link is shifted from the source base station to a target base station. This process of making the shift is commonly referred to in the radiotelephone communication area, or cellular communication area as a handover process.
A handover can be defined as a change of channel during a call, either because of degradation of the quality of the RF channel which includes, power level or communication link quality below a certain threshold, or because of the availability of another channel which can allow communication at a lower transmit power, or to prevent a mobile station from grossly exceeding the planned base station boundaries. A handover may occur during a call in progress (e.g. from a traffic channel to a traffic channel), or during the initial signaling during call set-up. The handover may be either from a channel on the source base site to another channel on a target base site or between channels on the source base site.
In digital radiotelephone systems--such as time division multiple access (TDMA) and code division multiple access (CDMA) systems, a mobile assisted handoff (MAHO) process is utilized. In MAHO, the mobile station is provided with a neighbor list of candidate base stations available for handover of the mobile unit communication signal from a source base site to a target base site. At intermittent times, the mobile unit will measure a signal quality parameter of transmissions from the neighboring base stations. The signal quality parameter may be signal strength, or another appropriate parameter such as energy per chip per total noise (EC/Io), bit error rate (BER), frame erasure rate (FER), or color code. These measurements are gathered to determine a preferred list of target base transceiver stations to which handover may be directed.
Despite handover capability, a communication link failure followed by a dropped call, may happen when the signal-to-noise (S/N) level drops quickly in either a link from the mobile unit to the base station, or vice versa, resulting in the mobile communication unit's inability to detect handover commands. In order to maximize mobile unit handover performance, system parameters such as handover neighbor lists, handover thresholds, and access criteria are manually tuned. This process, which requires significant time and manpower, is commonly referred to as system optimization.
For example, compilation of handover neighbor list associated with each base transceiver station, or base site, begins with a manual process of selecting neighboring base sites which may be appropriate. Next, upon initial system deployment, maintenance personnel must drive test the typical mobile communication unit travel routes of the entire system, in order to determine whether or not the optimal neighboring base sites were selected for each handover neighbor list. Unfortunately, the manual process of drive testing does not consider dynamic signal-to-noise (S/N) conditions which are impacted by frequency planning, traffic loads, and base station coverage changes. In addition, this manual tuning process and drive testing must be repeated each time frequency planning and/or replanning of base station coverage areas is introduced. Further, if inappropriate neighboring base sites are selected for handover neighbor lists, an increase in dropped calls results, thereby degrading overall system performance and adversely impacting customer service.
Moreover, methods to select appropriate neighboring base sites for handover neighbor lists that are, for example, based on a proximity of target base stations to a particular source base station, also fail to provide optimal handover neighbor lists. While somewhat effective, this process does not restrict the locations being considered as handover locations nor does it validate the coverage of a candidate target base station to meet a predefined signal quality.
Therefore, a need exists for an improved method for generating handover neighbor lists in a radiotelephone communication system, which overcomes prior art problems.